Often you can find a great bargin at a surplus store or at a flea market on un-marked zener diodes. This device here will not only test for good diodes but will tell you the zener voltage of that diode.

Zener diodes are used as voltage regulators and will begin to conduct in the reverse or "breakdown" mode when enough voltage is applied. This voltage will remain constant or "regulated".

To use this tester, supply enough voltage from a battery or power supply that will cause the highest voltage zener you might have to operate. If you might have some 24 or 48 volt types then you need that voltage as a minimum.

I use 12 volts on mine since most of the diodes I use operate below that voltage. Next turn the pot all the way down (zero volts).

Attatch the diode and begin to increase the voltage. Watch the meter and when it stops going any higher you have reached the "zener point" for that diode.

Now, just read the voltage on the meter and mark the diode. If the meter does not read as you increase voltage, try reversing the diode.

If that dosent work you have a shorted diode or it's not a zener diode.